Amplifier system



March 28, 1939. R. H, GEORGE ET AL 2,151,757

AMPLIFIER SYSTEM Filed Sept. 50, 1935 INVENTORS Roscoe [-1, Gear e y/Iowan! J 449 7 "ATTORNEY til Patented Mar. 28, 1939 UNITED STAT ES AIWILIFIER SYSTEM Roscoe H; George and Howard J. Heim, West Lafayette, Ind assignors to Radio Corpora tionof" America, a corporation of Delaware Application September 30, 1935, Serial No. 42,821

5 Claims.

This. invention. relates to radio frequency circuits and in particular to intermediate frequency amplifying circuits.

Briefly the, invention is based on the use. of a local fixed frequency oscillator upon which is transferred the modulating wave from a carrier frequency wave, thereby eliminating the difliculties experienced with "tracking a variable local oscillator particularly in heterodyne circuits. It is obvious that the overcoming of these difliculties is extremely valuable particularly at ultra-high frequencies.

Among the objectsof the invention are the following; to provide. waysand meansfor increasing the stability of radio frequency detector circuits; to provide a new intermediate frequency amplifying system; and to provide an intermediate frequency amplifying system to overcome certain difficulties inherent in superheterodyne systerms.

A further object of the, invention is to provide an intermediate frequency amplifier utilizing a local oscillator of fixed frequency.

A still further object of theinvention is to provide an intermediate frequency amplifier in which the, difficulties attendant the use of a variable local oscillator are overcome.

Still further objects and advantages of the invention will suggest themselves and become apparent to those skilled in the art upon reading the following specification in connection with the accompanying drawing of which Fig. 1 shows a schematic diagram of one embodiment of the invention, and

. Fig. 2 shows a schematic diagram of a modified embodiment of the invention.

Referring now to the drawing in Fig. l, the double-diode I, is used to rectify the incoming modulated radio frequency signals fed to it through the R. F. transformer 3. The input of the transformer 3 receives energy from a. R. F. signal amplifier (not shown) which'may be of any type well-known in the art. Suitable tuning such as the condenser 5, is placed, across the secondary of the transformer 3 to increase the voltage fed to the double-diode l. A center-tap l on the secondary of transformer 3 permits full wave rectification to take place. In series with the output resistor 9 is a'source of biasing voltage II and a coupling coil l3. The coupling coil I3 is coupled to the shielded stable I. F. oscillator l5 which is shown for example as an inductive feed-back oscillator for simplicity. It should be understood, however, that any other stable oscillator such as a piezo-electric, magneto-striction, or resistancestabilized' oscillator maybe used; the important feature of the oscillator being stability rather than the specific form of the circuit. A shield l1 surrounds the oscillator Hi to prevent stray radiation from affecting the operation of the system.

To further control the transfer of energy from the oscillator to the detector circuit, an electrostatic shield i9 is disposed between the coupling coil I3 and the output coil 2|. The use of the electrostatic shield l9 prevents stray electrostatic coupling between the coils l3 and 21 and insures that the variable coupling between these coils shall be purely inductive, permitting thereby improved operation.

The value of the voltage induced in the coil I3 is adjusted so that its peak value is just equal to the voltage source H which has its negative terminal connected to the center-tap l of the secondary of the transformer 3. Under these conditions and in the absence of R. F. signals, no current will fiow through the double-diode I or its output resistor 9, since the voltage supply H, elfectively-nullifies the voltage induced in the coil 1.3, because the positive peak of the induced voltage is cancelled by the negative voltage of source If now modulated R.F. signals are impressed on the input terminals of the transformer 3, the positive half cycles of the RF. energy will cause current to fiow through the double-diode I and its output resistor 9 whenever the algebraic sum of the instantaneous RF. voltage, the induced I.F. voltage, and the source I l is greater than zero. It will be appreciated that the amplitude of the voltage appearing across the resistor 9 will depend upon the depth of modulation on the RF. wave since this determines, in part, the instantaneous value of the R.F. voltage. Consequently, the amplitude of the rectified IF. voltage is modulated in the identical manner and degree as the RF. energy is modulated. The voltage appearing across the resistor 9 is composed therefore of the modulating wave, the rectified R.F. wave, and the rectified I.F. wave and this voltage is amplified by the IF. amplifier 23. The RF. energy will be suppressed by the II. transformer 25, or if it is desired a by-pass con denser of suitable capacity (not shown) may be connected across the resistor 9, the source I I, and coupling coil l3: so as to present a low impedance path to the RF. energy and a relatively high impedance path to the 1.15. energy. In general, however, the stray capacities to ground of the component parts of the circuit provides suficient filtering of the rectified RF. components to make additional capacity unnecessary.

The amplified, rectified, modulated LF. energy produced by the above described steps and apparatus may then be fed to additional I.F. amplifying stages (not shown) and then detected and utilized. The use of the LF. transformer 25 for transferring the amplified rectified voltage above described, serves to produce across the sec ondary of the transformer 25 pure modulated "track the tuning of the RF. stages as is required in superheterodyne systems. This simplification is of great advantage since it effectively reduces maintenance of the receiver and provides increased stability of operation. This last is particularly true in ultra-high frequency receivers where the local oscillators frequency becomes so high that unless some form of frequency control is used, frequency drift of the oscillator tends to prevent successful operation. The use of a frequency control element necessarily limits the local oscillator to a fixed frequency and in this respect, this invention is exceedingly well adapted.

Referring now to Fig. 2 which shows a modification of the invention and in which identical parts bear the same reference numbers as Fig. 1, it will be seen that the source H and coupling coil is of Fig. 1 is replaced by the resistor 2'1 and that the oscillator I5 is replaced by the oscillator 29. In this modified form, the modulated RF. energy fed to the double-diode I is rectified during the negative half cycle of the grid voltage of the IF. oscillator 29. During the positive half cycle of this voltage, the grid 3| draws current through the resistor 27, producing a potential drop which applies negative potential to the plates of the rectifier I, so as to nullify the positive half cycles of the modulated RF. voltage thereby causing cessation of the fiow of rectified current through the resistor 9. Thus the rectified modulated R.F. voltage is superimposed upon the LP. oscillator energy to produce modulated I.F. energy as explained above in connection with the description of Fig. 1. Since the remaining parts of Fig. 2 are identical with Fig. 1 further description is deemed unnecessary.

It is, of course, understood that other modifications of oscillators, rectifiers, or amplifiers may be substituted for those shown and/or described without departing from the spirit or scope of the invention.

Having now described our invention, What we claim is;

1. In receiving apparatus, a rectifier having an input and output circuit, means for producing local oscillations of intermediate frequency, a source of modulated radio frequency energy, means for impressing modulated radio frequency and constant intermediate frequency energies simultaneously upon the rectifier input, means for biasing the rectifier to cut-off value during 7 alternate half cycles of the intermediate frequency oscillation, means for producing in the output circuit of said rectifier modulated intermediate frequency signals representative of the modulated radio frequency impressed signals and a load circuit for utilizing said intermediate frequency signals.

2. The method of intermediate frequency amplification of radio frequency signals which comprises supplying modulated radio frequency energy to a detector circuit, locally and separately producing constant frequency-intermediate frequency energy, biasing the detector circuit to a predetermined operating condition, rectifying the modulated radio frequency energy intermittently and cyclically at a rate determined by the frequency of the locally produced intermediate frequency energy, filtering out of the produced rectified energy the radio frequency components, filtering out the direct current components from the produced rectified energy, amplifying the resultant modulated intermediate frequency energy and supplying the amplified energy to a load circuit.

3. The steps in the method of intermediate frequency amplification of radio frequency signals which comprises supplying modulated radio frequency energy to a detecting circuit, separately and locally producing constant frequency-intermediate frequency energy, biasing the detecting circuit to a predetermined operating condition, combining the supplied and locally produced energies in the detecting circuit and rectifying the modulated radio frequency energy intermittently and cyclically at the rate determined by the frequency of the locally generated intermediate frequency energy so that the output energy is derived from the detecting circuit only when the combined energies exceed a predetermined value.

4. The method of intermediate frequency amplification of radio frequency signals which com prises supplying modulated radio frequency energy to at detecting circuit, producing locally and separately constant frequency-intermediate frequency energy, biasing the detecting circuit to a predetermined operating condition, combining the supplied and locally produced energies in the detecting circuit, rectifying said energies intermittently and cyclically at a rate determined by the frequency of the locally generated intermediate frequency so that the output energy is derived from the detecting circuit only when the combined energies exceed a predetermined value, filtering out the radio frequency and direct cur rent components from the produced rectified energies, amplifying the resultant modulated intermediate frequency energy and supplying the amplified energy to a load circuit.

5. In receiving apparatus, a rectifier, means for separately and locally producing oscillations at intermediate frequency, means to receive modulated radio frequency energy, means to bias the rectifier to a predetermined operating condition, means for impressing the rectified modulated radio frequency energy and the separately developed constant intermediate frequency energy simultaneously to the rectifier, means for rectifying the impressed energies intermittently and cyclically at the rate determined by the frequency of the locally generated intermediate frequency so that the output energy is derived from the detecting circuit only when the sum of the energies exceeds a predetermined value, and means to derive from the rectified energies intermediate frequency signals energy representative of the modulated radio frequency energy.

ROSCOE H. GEORGE. HOWARD J. HEIM.

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